The present invention generally relates to fuel pressure control apparatus for cylinder injection engine, and particularly to a fuel pressure control apparatus for cylinder injection engine which is suited to control the fuel pressure in the range from the start of engine to the stationary state, the fuel pressure at the time of the transient from the stationary state, and the fuel pressure at the time when fuel is cut.
The conventional fuel pressure control apparatus for cylinder injection engine has a high-pressure fuel pump to be driven by engine, and a low-pressure fuel pump provided on the upper stream side of the high-pressure fuel pump in order that the high-pressure fuel can be directly injected into combustion chambers through high-pressure fuel injection valves that are provided in the respective combustion chambers of the engine. In addition, the fuel pressure in the pipes at the high-pressure fuel injection valves is controlled by fuel pressure adjusting means which makes feed back control so that the actual fuel pressure measured by fuel pressure detecting means can be coincident with a target value that is optimum to the engine speed.
In this conventional fuel pressure control apparatus for cylinder injection engine, however, since the fuel pressure is always under the feed-back control, the discharge pressure of the fuel pump driven by the engine is greatly affected by the cranking speed of engine and combustion state. Particularly, at the time of start, it is difficult to control the fuel pressure by feed back. Even if the feed back control is carried out, hunting or the like is caused to make the fuel pressure control the more unstable.
An example of the fuel pressure control apparatus with the above drawback removed is proposed as for example disclosed in JP-A-5-149168. In this proposed technique, different fuel pressure control systems are respectively used at the start when the discharge pressure of the high-pressure fuel pump driven by engine is unstable and at the normal driving condition. In this case, an actual fuel pressure signal is supplied to a fuel pressure regulator of the high-pressure fuel system, and a control system makes feed-back control in order to cause the actual fuel pressure value to coincide with a target fuel pressure value. This control system includes start discriminating means for deciding whether the engine starts, and fuel pressure control extent calculating means which supplies a fuel pressure signal for that control extent according to the target fuel pressure value when the engine starts.
In the above fuel pressure control apparatus, at the normal driving time, or only when the discharge pressure of the pump based on the engine rotation is stable, the fuel pressure is controlled in a feed-back manner, while at the start when the discharge pressure of the pump is unstable the fuel pressure is controlled in a feed-forward manner by a fuel pressure signal according to only the target fuel pressure value as a fixed amount of control, thereby stabilizing the fuel pressure control.
However, this conventional fuel pressure control apparatus is constructed to stop the feed back control when the fuel pressure in the pipes at around the fuel injection valves is most greatly changed, or at the start time. When the engine starts at a low temperature, the voltage of the battery as a driving power supply is reduced, sometimes making even the operation for driving the feed-forward control unstable. Thus, even if the fuel pressure in the pipes at around the fuel injection valves is controlled in a feed-forward manner by the fuel pressure signal according to only the target fuel pressure value as a fixed amount of control, it is not possible to assure the stabilization of the fuel pressure control at the time of start. In addition, at the time of start, it is necessary to raise the fuel pressure in the pipes at around the fuel injection valves as soon as possible, and make the following control operation. The above proposed technique does not consider these points.
Moreover, it is necessary that the fuel pressure feed-back control consider the change of the target fuel pressure of engine due to the abrupt change of driving conditions at the transient or the like. In other words, when the target value is abruptly changed, or when the fuel pressure is, for example, instantaneously increased and then decreased, a proper amount of control for feed back is not given, so that the actual fuel pressure sometimes overshoots or undershoots. These defects will be ascribed to the control ability in transients. The above proposed technique considers neither the detailed control to take when the target fuel pressure is suddenly changed, nor the detailed method of setting the amount of reference control as the amount of feed-forward control.
Moreover, in this type of fuel pressure control apparatus, foreign matter such as abrasion powder or dust enters into the pipes or the regulator of fuel pressure control means, causing troubles in the pipes and control equipment. The foreign matter must be removed from those places.